1. Field of the Invention
The present invention relates to an oscillator circuit having an amplifier circuit for amplifying the oscillation output from a piezoelectric oscillator such as a quartz oscillator and to an integrated circuit for oscillation. More particularly, the invention relates to an oscillator circuit adapted for high-frequency operation and to an integrated circuit for oscillation.
2. Description of the Related Art
Conventionally, an oscillator circuit equipped with an amplifier circuit for amplifying the oscillation output from a piezoelectric oscillator such as a quartz oscillator as shown in FIG. 17 is available. In this circuit, a quartz oscillator XL is connected between the input terminal in and the output terminal out of the CMOS inverter INV acting as an amplifier portion. Also, a feedback resistor Rf is connected between these input and output terminals. The input terminal in and output terminal out are connected with a power-supply terminal VSS (0 V) via capacitive elements CG and CD, respectively, serving as load capacitors.
Today, oscillator circuits are required to operate at higher frequencies. As the operating frequency is made higher, the size of the quartz oscillator decreases. Concomitantly, the oscillation output, i.e., the electric current (hereinafter may be referred to as xe2x80x9cquartz currentxe2x80x9d) flowing through the quartz oscillator, becomes weaker. Therefore, in the configuration of FIG. 17, oscillation is possible for a quartz oscillator up to where a 100 MHz third overtone is used as the oscillation frequency. However, at still higher frequencies exceeding 120 MHz, for example, the amplification factor of the amplifier portion is small and so the oscillation output cannot be amplified sufficiently. Consequently, it has been impossible to operate the circuit as an oscillator circuit.
Where one attempts to produce higher frequencies by making use of overtones higher than the third overtone, the following problems take place. The configuration of FIG. 17 is integrated except for the quartz oscillator XL. As shown in the equivalent circuit of FIG. 18, the quartz oscillator XL can be represented as a series circuit of a resistor RX and an inductor LX. The other components in the form of an integrated circuit can be represented as a series circuit of a negative resistor RL and a capacitor CL. The frequency characteristics of the negative resistor RL are shown in FIG. 19, where the resistance value is plotted on the vertical axis and the frequency on the horizontal axis. Thus, the negative resistance RL of the third overtone oscillation is indicated. The values of resistive components of the quartz oscillator at the fundamental wave, third overtone, and fifth overtone are indicated by R01, R03, and R05, respectively. If the negative resistance RL is greater than that of the resistance RX in the negative direction, the circuit operates as an oscillator circuit. If a quartz oscillator of 30 MHz is used to generate the third overtone in the configuration of FIG. 17, the negative resistance RL decreases with frequency from a peak close to 30 MHz that is the oscillation frequency of the third overtone as shown in FIG. 19. For example, at the oscillation frequency of 50 MHz of the fifth overtone, the negative resistance RL is smaller than the resistive component of RX of the quartz oscillator and, therefore, the oscillator circuit cannot be set into operation. For this reason, a coil LADD and a capacitive element CADD are sometimes connected between a capacitive element CD and a power-supply terminal VSS as shown in FIG. 20 to produce the fifth overtone. However, it is necessary to provide the external coil LADD and capacitive element CADD, which in turn increases the circuit area. In addition, it is laborious to control the values of these added elements.
Accordingly, it is an object of the present invention to provide an oscillator circuit which is adapted for a piezoelectric oscillator which generates a weak oscillation output. The provided oscillator circuit operates at increased speed. It is another object of the invention to provide an integrated circuit which is used for oscillation and which is used to construct the above oscillator circuit.
An oscillator circuit in accordance with the present invention comprises: an amplifier portion consisting of plural CMOS inverters connected in cascade; a piezoelectric oscillator connected between the input and the output terminals of the amplifier portion; a first load capacitor connected between the input terminal of the amplifier portion and a terminal at a certain potential; a second load capacitor connected between the output terminal of the amplifier portion and the terminal at the certain potential; and a filter circuit included in the amplifier portion. The CMOS inverters have gate areas which decrease successively in going from the first stage to the last stage.
Preferably, the filter circuit is defined such that the negative resistance of the circuit formed by the amplifier portion, the feedback resistor, the first and second load capacitors has a peak at a given frequency.
Preferably, the filter circuit described above is in a signal path between the input and the output terminals of the amplifier portion. A capacitive element is connected with the input terminal of at least one of the above-described plural CMOS inverters. A resistor is connected between the input terminal of the CMOS inverter with which the capacitive element is connected and the output terminal.
The above-described filter circuit is preferably composed of a capacitive element and a resistor. In the preferred embodiment shown in FIG. 1, the capacitive element is connected between the input terminal of the amplifier portion and the input terminal of the first-stage CMOS inverter of the amplifier portion. The resistor is connected between the input and output terminals of the first-stage CMOS inverter.
In the above-described amplifier portion, the CMOS inverters desirably have the same channel length. The CMOS inverters have channel widths that decrease successively in going from the first stage to the last stage.
Another oscillator circuit in accordance with the present invention comprises: an amplifier portion consisting of first, second, and third CMOS inverters connected in cascade, said CMOS inverters having gate areas that decrease successively in going from the first stage to the last stage; a filter circuit consisting of a capacitive element and a resistor connected between the input and the output terminals of the first CMOS inverter, the capacitive element having one terminal connected with the input terminal of the first CMOS inverter and the other terminal acting as the input terminal of the amplifier portion; a piezoelectric oscillator connected between the input and output terminals of the amplifier portion; a feedback resistor connected between the input terminal of the amplifier portion and a terminal at a certain potential; and a second load capacitor connected between the output terminal of the amplifier portion and the terminal at the certain potential.
Preferably, each of the above-described oscillator circuits has a resistor connected to at least one of first and second signal paths to suppress the electric current flowing through the piezoelectric oscillator. The first signal path is formed by the input terminal of the amplifier with which the first load capacitor is connected and by the certain potential. The second signal path is formed by the output terminal of the amplifier with which the second load capacitor is connected and by the certain potential.
In each of the above-described oscillator circuits, the sources of the MOS transistors forming the CMOS inverters of the amplifier portion and the terminal at the certain potential are preferably connected with a power-supply potential via a current-limiting device. Desirably, the current-limiting device connects some of the plural MOS transistors in parallel. The others of the plural MOS transistors are all turned on under an initial condition of the oscillation output delivered from the output terminal described above. As the oscillation output becomes more stabilized, the MOS transistors are turned off successively.
Preferably, the aforementioned piezoelectric oscillator of the oscillator circuit is a quartz oscillator and effects overtone oscillation.
Preferably, in each of the oscillator circuits, an output terminal for producing an output signal to the later-stage circuit is connected with the output terminal of the CMOS inverter in a stage immediately preceding the last stage of the amplifier portion.
Preferably, in each of the oscillator circuits, one input terminal of a differential amplifier circuit is connected with the input terminal of the CMOS inverter in a stage immediately preceding the last stage of the amplifier portion. The other input terminal of the differential amplifier circuit is connected with the output terminal of this CMOS inverter. An output signal for a later-stage circuit is delivered from the output terminal of the differential amplifier circuit.
Preferably, the components of the oscillator circuit excluding the piezoelectric oscillator are built as an integrated circuit for oscillation. More preferably, the capacitive elements are intermetallic capacitors, and the resistors are thin-film resistors.
The above and other objects, aspects, features and advantages of the invention will be more readily apparent from the description of the preferred embodiments thereof taken in conjunction with the accompanying drawings and appended claims.